This invention relates to switching power converters.
As operating speeds of electronic systems have increased, so have the performance demands imposed upon system power sources. Voltages supplied to individual active components in high performance electronic assemblies must be maintained within relatively narrow ranges (for example, +/-5%) even though the current drawn by any device may change by as much as an order of magnitude in a fraction of a microsecond. Maximum performance is demanded of the power source when a large number of devices change state essentially simultaneously. Under these circumstances, the relative power drawn from the power source by the circuit assembly can change from 10% to 100% of full load, and aggregate current slew rates can exceed 20 Amperes-per-microsecond. Trends toward higher speed devices which operate at lower voltage levels can be expected to further increase the rate-of-change of current which must be accommodated by the power source.
Known modular DC-DC switching power converters are intended to deliver an essentially constant voltage, Vo, to a load. Such converters include switching circuitry, control circuitry and an output filter. The switching circuitry, which includes one or more switching devices, converts energy from an input source into a periodic pulsating voltage waveform (at a converter operating frequency which may either be fixed or variable) which is delivered to the output filter. The average value of the pulsating voltage waveform can be varied by means of control signals delivered to the switching circuitry. The output filter, which consists of an output inductor (Lo) in series with an output capacitor (Co), is characterized by a first breakpoint frequency, ##EQU1## which is set well below the lowest operating frequency of the converter. By connecting the load across the output capacitor, the output filter delivers the average value of the pulsating voltage waveform to the load but attenuates the large time-varying component of the pulsating waveform so that its contribution to the output voltage (i.e. the output ripple) is small in comparison to the desired value, Vo, of the load voltage. By sensing the voltage across the load, and adjusting the control signals delivered to the switching circuitry, the control circuitry attempts to maintain the output voltage at a constant value, Vo, as the input source and load vary. Converters so described are generally self-contained assemblies with terminals (e.g. pins, wires) for making electrical connections to the input source and the load. In some cases the converter is mounted directly on the electronic assembly which forms the load; in others it may be physically separate from the assembly.
Besides acting as an element for reducing output ripple, the module output capacitor is also intended to act as an energy storage medium for minimizing output voltage variation in response to sudden changes in load current. In principle, the peak output voltage deviation in response to large changes in load current can be controlled through appropriate sizing of the output capacitor. In practice, however, this is not the case, since some parasitic inductance unavoidably exists in the interconnections between the output capacitor and the load. The value of this parasitic inductance will depend on the geometry of both the interconnections between the output capacitor and the converter output terminals (Lp), and the interconnections between the converter terminals and the load (L1). When the current drawn by the load exhibits a high rate-of-change, a voltage drop equal to the total parasitic inductance, Lp+L1, multiplied by the rate-of-change of current will develop across the parasitic inductance. Since the output capacitor voltage cannot change instantaneously, the load voltage will be reduced in an amount equal to the voltage drop across the parasitic inductances, and, for a rapid change in load, this voltage deviation can be quite significant. For example, a rate-of-change of load current of 20 Amperes-per-microsecond will produce a 2 volt drop across 100 nanohenries of parasitic inductance. In a 5 Volt system, this would represent 40% deviation in load voltage. To achieve a more acceptable deviation of 0.1 Volt would require that the total parasitic inductance be lower than 5 nanohenries.
One prior art method of attempting to overcome the effect of the parasitic inductances is to place additional capacitance, outside of the converter, across the load. This creates several problems. First, by increasing the aggregate output capacitance, the response time of the converter is degraded. Second, use of too much capacitance may cause closed-loop instability of the converter. Finally, if small amount of external capacitance are used, a rapid change in load current will excite an oscillatory ringing of the load voltage. This ringing, which is associated with the circuit loop formed by the external capacitor, the output capacitor internal to the converter, and the parasitic inductances, can be relatively large in comparison to the nominal output voltage.
The effects of parasitic inductance on load voltage result directly from the placement of the output capacitor within the modular converter. As was intended, the internal output capacitor acts as an essentially noncompliant voltage source when the converter is confronted with changes in load current. Unfortunately, instead of limiting the compliance of the voltage across the load, it limits the compliance of the voltage which feeds the parasitic inductances interposed between the capacitor and the load. If no external capacitor is used with a prior art converter, the noncompliance of the voltage across the internal output capacitor exposes the load to the full effect of the voltage drop induced in the parasitic inductances by rapid changes in load current. Whether or not an external capacitor is used, the presence of an internal output capacitor acts as a barrier to corrective control response. If, for example, the converter module responds to a change in load by increasing the current flowing in the module output inductor, the voltage slew rate limitation imposed by the internal output capacitor will delay a similar increase in current flowing in the parasitic inductances toward the load.